The present invention generally relates to video signal discriminating apparatuses, and more particularly to a video signal discriminating apparatus for discriminating whether or not a video signal exists.
A video signal superimposing circuit is conventionally used to carry out a superimposed display on a television receiver, for example. The video signal superimposing circuit is supplied with a main video signal received from a television broadcasting station, for example, and a sub video signal received from a personal computer which is synchronized to the main video signal, for example. The video signal superimposing circuit outputs a superimposed video signal by replacing the main video signal by the sub video signal only during a time period in which the sub video signal is related to sub information such as characters, figures and the like.
When carrying out such a superimposed display, the sub video signal is constantly obtained from the personal computer. On the other hand, no main video signal is obtained when the receiving channel of the television receiver is changed to an unused channel in which no broadcasting is made. In addition, in the case where a video tape recorder is used instead of the television receiver, no main video signal is obtained when an operating mode of the video tape recorder is set to a stop mode.
Since the superimposed video signal comprises the main video signal and the sub video signal only during the time period in which the sub video signal relates to the sub information, a composite synchronizing signal of the superimposed video signal is obtained from the main video signal. Accordingly, no composite synchronizing signal exists in the superimposed video signal when no main video signal is obtained, and in this case, a reproduced picture obtained from the superimposed video signal becomes abnormal.
Accordingly, a video signal discriminating circuit for discriminating whether or not a video signal exists was previously proposed in a Japanese patent application No. 59-209317 filed Oct. 5, 1984 in which the applicant is the same as the assignee of the present application. According to the previously proposed video signal discriminating circuit, a discrimination is carried to determine whether or not the main video signal exists based on a counted value which is obtained by counting pulses in the main video signal during a time period in which a vertical synchronizing pulse of the main video signal exists.
However, since the previously proposed video signal discriminating circuit simply counts the pulses in the main video signal during the time period in which the vertical synchronizing pulse exists, the discrimination cannot be carried out with a satisfactory accuracy. In addition, in the case where a signal-to-noise ratio of the main video signal becomes deteriorated, there is a problem in that the discrimination accuracy becomes poor. Furthermore, there is a problem in that a sub video signal which is constantly in stable synchronism with the main video signal is required to detect the time period in which vertical synchronizing pulse of the main video signal exists.